1. Field of the Invention
The present invention relates to a semiconductor integrated capacitive acceleration sensor and relative fabrication method.
2. Discussion of the Related Art
As is known, acceleration sensors are used widely in the car industry for airbags, ABS, active suspensions, engine control and ASR (Anti Slip Rotation). In recent times, electromechanical silicon microstructures fabricated using microelectronics technology have been proposed for use as acceleration sensors, in view of the numerous advantages afforded as compared with traditional macroscopic inertial mechanical switches. The advantages include low cost, high degree of performance and reliability, better signal/noise ratio, integration with memory circuits for forming intelligent sensors and on-line self-test systems, and greater reproducibility and sensitivity.
Prototype silicon-integrated acceleration sensors are currently fabricated in six types substantially differing as to operation;
a) piezoelectric: a mass, moved by acceleration, compresses or stretches a thin film of piezoelectric material, across which a voltage may be detected;
b) piezoresistive: inertial displacement of a silicon diaphragm unbalances a Wheatstone bridge comprising piezoresistive elements diffused in the diaphragm;
c) capacitive: acceleration induces displacement of a seismic mass forming the movable electrode of a single capacitor (absolute variation in capacitances), or of an electrode common to two electrically connected capacitors to vary the two capacitances in opposite directions (differential variation in capacitance);
d) threshold: acceleration-induced inflection of a silicon microbeam closes an electric circuit;
e) resonant: acceleration shifts the intrinsic frequency of an oscillated suspended micromechanical structure; PA1 f) tunnel-effect: acceleration varies the distance between two electrodes, one of which is movable, and hence the quantic tunnel current.
The present invention relates to a differential capacitive acceleration sensor.
Traditionally, integrated microstructures have preferably been fabricated using the bulk micromachining technique, whereby a silicon wafer is processed on both faces to exploit the excellent mechanical properties of monocrystalline silicon. Front-rear processing, however, and the need for particular handling of the wafers made bulk micromachining incompatible with current integrated circuit fabrication technology.
In the mid-80's, surface micromachining was therefore proposed, whereby the sensitive element is formed of polycrystalline silicon, and suspended structures are formed by depositing and subsequently removing sacrificial layers. Details of this are to be found, for example, in the article by W. Kuehnel and S. Sherman entitled "A surface micromachined silicon accelerometer with on-chip detection circuitry" in Sensors and Actuators A 45 (1994) p. 7-16, and in Patent EP A-O 543 901 filed by Analog Devices, Inc. Though compatible with planar microelectronics technology, the solutions described pose serious problems when releasing the suspended structures (i.e. detaching them from the rest of the semiconductor body) due to the tendency of the micro structures to collapse as a result of the capillarity and Van de Waals forces involved. (Details of this are to be found, for example, in "Stiction of surface micromachined structures after rinsing and drying: model and investigation of adhesion mechanisms" by R. Legtenberg, H. A. C. Tilmans, J. Elders and M. Elwenspock, Sensors and Actuators A, 43 (1994), p. 230-238).
Other highly specialized techniques, such as "wafer dissolving," provide for forming silicon microstructures by means of dedicated processes which are totally incompatible with standard planar microelectronics technology. In a sense, these "ad hoc" processes simply consist of transferring on to silicon what is currently done using other materials, and only provide for fabricating the sensitive portion, so that the processing and control circuit must be formed on a separate chip.
For sensors of a different type, dedicated SOI (Silicon-on-Insulator) substrates have been proposed, wherein the starting wafer comprises a stack of Silicon-Silicon Oxide-Silicon with the oxide selectively removed at the sensor area to form an air gap. The trenches formed from the front of the wafer after contacting the air gap provide for forming the suspended structure. Details of this, relative to a shear stress sensor, are to be found, for example, in the article entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology" by J. Shajii, Kay-Yip Ng and M. A. Schmidt, Journal of Microelectromechanical Systems, Vol. 1, N. 2, June 1992, p. 89-94. The bonding technique used (excluding formation of the air gap) is also described in this article entitled "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J. Hausman, G. A., Spierings, U. K. P. Bierman and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, N. 8, August 1989, p. 1426-1443.
It is an object of the present invention to provide an acceleration sensor and relative fabrication method, designed to overcome the drawbacks typically associated with currently proposed solutions.